Muffler with inner sound-absorbing tube



Aug. 1, 1950 R. B. BOURNE 2,516,949

MUFFLER WITH INNER SOUND ABSORBING TUBE Filed Feb. 9, 1948 INVENTOR Patented Aug. 1, 1950 MUFFLER WITH INNER TUB SOUND -ABSORBING E Roland B. Bourne, West Hartford, Conn, assignor to The Maxim Silencer Company, Hartford, 601111., a corporation of Connecticut Application February 9, 1948, Serial No. 7,258

1 Claim.

structions disclosed in my prior Patents Re. "22,283, 2,271,892 2,326,612 give very uniform attenuation over a wide band of frequencies, in the larger sizes (which may reach two feet and over in diameter) the attenuation in the higher frequencies suffers.

The present invention uticrease the attenuation of higher frequencies.

The invention will now be described with relation to the accompanying drawing, in which Fig. 1 is a view of the complete assembly; and

Fig. 2 is an enlarged view, in median section, of one of the end sections shown in Fig. 1.

The silencer is shown in Fig. 1 on a greatly reduced scale, the particula silencer represented having a pipe diameter of twenty eight inches and an over-all length of more than forty four feet. The silencer is built in three sections 1, 2 and 3, sections and being of identical construction and section 2 being of substantially the construction shown in Patent 2,326,612 with the addition of a central resonance chamber. This section has inlet and outlet conduits 4 and 5 of perforated metal, each being surrounded by a perforated metal sleeve 6 for a portion of its length. This sleeve terminates short of a partition 5' leaving room for an acoustic coupling i into a surrounding chamber 8. The acoustic coupling is preferably made as shown by providing holes in the conduit, as this has the advantage over a slot of providing support for the conduit. The space between the conduit and the sleeve is filled with a sound absorbing material 9 such as copper wool. This type of construction is fully described in the patents referred to. Between these two end sections is a central conduit in which is imperforate except for coupling holes ii entering an annular chamher [2. This section is a volumetric resonator, and acts to attenuate sounds of a frequency for which the resonator is tuned. The dimensions of the chamber will of course be varied to cause the resonant frequency to correspond to one ex- 2 isting in the system to which the silencer is attached.

The two end silencers I and 3 are identical except for position, and each comprises a casing is provided with end walls [4 and partitions l5, l6, and H, which are provided with holes as will be described to receive removably the main sound conducting conduit. At each end each silencer is provided with a coupling flange l8 for attachment to the next unit in the acoustical line, and with a header is secured by bolts 2!] to the end wall [4. The flanges and headers are secured together by sleeves 2| having welded or otherwise secured to their inner ends short skew collars 22. A second concentric skew collar 23 is fixed as by welding to the inner surface of flange l9 concentric with the first. Similar pairs Of skew collars 24 and 25 are secured to each side of the partition [1 around the hole 26 through which the main channel passes. Parallel cylinders 2! and 28 of perforated metal pass freely through holes in the partitions l5 and I5, and have their ends received respectively outside the flanges 22 and 24 and inside flanges 23 and 25. Similar cylinders 29 and 3!] are located between partition l1 and the end wall M.

The space between the concentric cylinders of perforated metal is filled with a pervious layer of sound absorbing material such as copper or bronze wool, although other sound absorbents can be used if regard be had to the condition of the exhaust gases. As is fully discussed in the patents referred to, the resonating action of the outer chambers 32, 33, 34 and 35 causes not only the direct attenuation of sound waves corresponding to the resonating frequencies, but causes high attenuation of all frequencies by reason of the disposition of the sound absorbing material at a point of high acoustic velocity thereby causing maximum dissipation of the acoustic energy in the form of heat.

In the present invention the dissipation of high frequencies is further increased by the arrangement of the main channel in reversely slanting sections. I have discovered that where the pipe size is large there is a strong tendency for the higher frequencies to travel through it in a beam and to resist being drawn outwardly through the pervious layer of sound absorbent. This tendency is present in all pipe sizes, but becomes more consequential in the larger sizes both because the ratio of cross sectional area to circumference increases with the pipe diameter, thereby decreasing the amount of sound absorbent available per unit of cross sectional area; and because the beam formation is effective on lower frequencies as their wave lengths become comparable with the pipe size.

By inclining the main channel, and particularly by reversing its inclination at partition 11, any tendency for sound waves to form a beam traveling straight down the channel is utilized to increase theattenuation, for. instead of changing their direction at the conduit surface the waves pass into and through the sound absorbent. A

The steeper the inclinationthe greater the effect, but also the greater the increase in back. pressure due to the abrupt change in direction of exhaust gas flow.

keep the angle of inclination as low as is compatible with the attenuation needed. In general an angle of Mi /2 with the axis has been found practically suitable. It will be observed that, assuming initial flow parallel to the axis, change direction causing dissipation of beamed energy occurs at the entrance to silencer i, at the in- 't'er'mediate bend'in the conduit, within conduit insilencer 2 at the entrance to silencer 3, and

at the intermediate bend in it's conduit. This particular absorption of sound is in addition to tion" the method" of supporting the perforated 'metal cylinders on the skew flanges permits ready disassembly so that the sound absorbent can be removed for cleaning or replacement. By removing. bolts 20 the flanges 18 and I0, together :with' the skew cylinders or collars attached to In instances where the backv pressure is of great importance, it is desirableto them, can be withdrawn in a direction along the axis of the cylinders 21, 28; leaving the latter free for unobstructed removal.

What I claim is:

A sound attenuating device comprising a cylindrical casing, an acoustically pervious wall of sound absorbing material defining within itself a sound conducting channel passing through casing in a bent line having its legs at an angle to the axis of the casing, said Wall comprising two pairs of concentric perforated metal shells and sound absorbing material between the shells, a partition within the casing located at the bend in said channel and provided with a hole aligned with the channel, two pairs of skew collars secured to the partition to support one end of each of the pairs of shells, a removable header at each end of the casing, and a pair of skew collars secured to the inner surface of each removable header to support the second ends of said shells, whereby the shells and sound absorbing material can he removed for cleaning or replacement.

ROLAND B; BOURNE.

REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATSS PATENTS Number Name Date- Re. 22,283 Bourne Mar. 9'; 1943 1,972,065 Noblitt Aug. 28, 1934: 2,016,253 Noblitt Oct. 1, 1935 2,04r6,193 Spicer June 30, 1936 2,270,825 Parkinson Jan. 20,, 1942 2,273,092 Dole Feb. 1'7, 1942 2,326,612 Bourne Aug. 10, I943 FOREIGN PATENTS Number Country Date 49,088 France Nov. '7, 193B 

